Hybrid electric vehicles that include both an internal combustion engine and an electrical machine operable as a motor to at least assist in driving the wheels of the vehicle are becoming more popular. This is at least in part due to their perceived greater fuel efficiency. The drivetrains in such vehicles, that is the collection of components through which power is conveyed to drive the vehicle, can be arranged in several layouts. Examples of two existing types of drivetrain layout used in hybrid electric vehicles are series drivetrain layouts and parallel drivetrain layouts.
FIG. 1 shows a series drivetrain layout 10 in simplified diagrammatic form. With reference to FIG. 1, an engine 20 is mechanically coupled to a first electrical machine 30 to rotate the rotor of that machine so as to allow that machine 30 to be operated as a generator. The first electrical machine 30 is electrically coupled to a second electrical machine 40 so as to allow that second electrical machine 40 to be operated as a motor. The second electrical machine is mechanically coupled to a transmission 50 such that the rotor of that machine drives the transmission 50. The transmission 50 is mechanically coupled to wheels 60 so as to drive those wheels 60. The series layout 10 further includes a battery 70 that is arranged to store excess electrical energy when the power output of the first electrical machine 30 exceeds the power consumption of the second electrical machine 40, and to supply stored electrical energy when the power output of the first electrical machine 30 is lower than the desired power consumption of the second electrical machine 40. The battery 70 may also store electrical energy generated by the second electrical machine 40 when that machine is operated as a motor in regenerative braking.
FIG. 2 shows a parallel drivetrain layout 100 in simplified diagrammatic form. With reference to FIG. 2, an engine 120 is mechanically coupled through a first clutch 130 and then a driveshaft 135 to transmission 140. The transmission 140 is mechanically coupled to wheels 150. The drive shaft 135 is also mechanically coupled to a second clutch 160, which is in turn mechanically coupled to an electrical machine 170 operable as a motor/generator. The motor/generator 170 is connected to a battery 180 so as to charge the battery 180 when operating as a generator and discharge the battery 180 when operating as a motor. The two clutches 130, 160 can be selectively engaged and disengaged such that:
(a) the engine 120 drives the wheels 150 and drives the motor/generator 170 as a generator to charge the battery 180;
(b) both the engine 120 and the motor/generator 170 drive the wheels 150, with the motor/generator 170 operating as a motor and discharging the battery 180
(c) the motor/generator 170 only is coupled to the wheels 150 and drives the wheels 150, again operating as a motor; and
(d) the motor/generator 170 only is coupled to the wheels 150 and is driven by the wheels to operate as a generator in regenerative braking.
Whilst a hybrid electric vehicle incorporating a series drivetrain layout may give improved efficiency in some operating conditions, it is also likely to be under-powered. This is because power to the wheels is provided by the motor and so vehicle performance is limited by the power output of that motor.
By contrast, in a hybrid electric vehicle with a parallel drivetrain layout, the motor/generator can be operated as a motor to assist the engine in driving the wheels, thereby increasing power to the wheels and the vehicle's performance. This arrangement, however, tends to be less efficient in some operating conditions.
It is therefore an object of this invention to provide a new driveline layout which seeks to maintain some of these advantages whilst addressing some of these drawbacks.